Electron emitter



Nov. 1, 1960 N. ANTON ELECTRON EMITTER Filed Dec. 28, 1954 IN V EN TOR.MCWOZ/LS 4/v7o/v United tes atcflfQ ELECTRON EMIITER Nicholas Anton,1226 Flushing Ave., Brooklyn, N.Y.

Filed Dec. 28, 1954, Ser. No. 477,983"

20 Claims. (Cl. 313-54) The present invention relates to electronemitters and particularly to an emitter in which atomic energy isutilized to produce the electrons.

At the present time, cathodes for electron discharge tubes such asvacuum tubes and the like, are generally either thermionic orsecondary-electron emitters and these cathodes are operated in closedvessels whichare either under vacuum or are filled with special gasesat.

predetermined pressures. in such closed vessels there is also provided asecond electrode or anode which is operated at a positive potential anddirects the action of the electrical field between the anode and theemitter.

The emitted electrons are attracted to the plate, constituting a currentflow, which may be controlled by one or more other electrodes in wellknown manner.

Thermionic emission is the emission of electrons from a metal which hasbeen heated, generally to incandescence.

In such emission an electron about to leave the metal induces a positivecharge on the surface behind it, this action giving rise to apotentialbarrier which must.

be overcome before electrons can escape to become free electrons inspace. The value of potential of the barrier is commonly given in termsof electronvolts and is called the work function of the metal. In orderfor electrons to penetrate the potential barrier and escape from themetal surface they must possess a minimum energy equal to theworkfunction. At room temperature the energy of electrons is much lessthan thework function, and consequently in conventional cathodes it isonly by heating the metal that the electrons vacquire suflicientadditional energy to escape from the metal surface. I

Secondary emitters depend upon electromagnetic radiation or high energyelectrons for the release of free electrons. When radiation or highenergy electrons are in- 2,958,798 Patented Nov. 1, 1960 plications,where it is desirable to provide a constant and sufficient rate ofemission, it is necessary to provide special means for maintainingconstant emitter temperature.

Secondary emitters have long life and no problem of heat dissipationis'involved. Nevertheless, these emitters also have disadvantages, amongwhich may be mentioned the dependence of the emission of secondaryelectrons on the quantity and intensity of the incident primaryradiation or the primary electrons, and the fact that the yield ofsecondary electrons per incident electron is relatively small.

The atomic emitter of the present invention overcomes the disadvantagesassociated with thermionic and secondary emitters. This emitterpossesses an indefinite life, has a constant rate of copious electronemission, can be operated at any desired temperature, either in a vacuumor in any gaseous atmosphere, and does not require and external sourceof power for heating or of incident radiation.

The atomic emitter of my invention comprises but two essentialcomponents: a radioactive material of long life which emits beta oralpha particles or both, and a semiconductor (as used herein, the termsemi-conductor means a material which under the band theory ofsolidstate physics has all of its inner bands filled and its conductionbands only partially filled above absolute zero temperature. ionmobility or positive charge movement therewithin, sometimes described interms of the movement of holes within the material and which .issubstantially ionized under the influence of incident sub-atomic chargedparticles without permitting the free conduction of ions therein). Theradioactive material is supported on a base serving as an electrode ormay be self-supporting, and the semi-conductive material is in turnclosely associated with the radioactive material, as by being depositedor laminated upon it. Beta or alpha particles emitted by the radioactivematerial pass through the semi-conductive material and in so doingexpend their energy in ionizing the molecules of that material andretage that their life is limited ,due to the evaporation of theemissive coatings at high temperatures. Moreover,- due to thatevaporation, contamination of the other tube elements occurs duringoperation. Also, due to the high temperature conditions, the filamentsused to heat the emitting coatings often burn out.

Another disadvantage of such thermionic emitters is that there arelimited conheat developed in the thermionic emitter. In some apleasing anumber of free electrons. The thickness of the semi-conductive materialis preferably chosen to have at least a minimum value such that the betaor alpha particles emitted by the radioactive material cannot emergetherefrom to produce high elficiency and low noise level.

An electric field is caused to act on the atomic emitter so thatscondary electrons generated at or near the interface of the radioactivematerial and the semi-conductive material by action of the radioactiveparticles emitted by the radioactive material will be acted upon by theelectric field and will be urged toward the outer surface of thesemi-conductor. Since a thin layer of amorphous or crystallinedielectric or semi-conductor when polarized by the action of an electricfield acts like a dense layer of heavy gas, these secondary electronsunder the accelerating action of the field will further ionize moleculesof the semi-conductor, thus generating additional free electrons. Theseadditional free electrons are in turn accelerated by the field and theaction is repeated resulting an an avalanche of electrons like aTownsend avalanche in a gas, and this avalanche ultimately breaksthrough the surface of the semi-conductor to become free electrons ofrelatively low energy which by proper design can be made just sufiicientto provide a cloud source of electrons just outside the emitter, similarto the space charge of a thermionic emitter, but with the desirableproperties mentioned above. The electron amplification thus produced iscomparable to that obtained in Geiger counters and each electronliberated by the original ra- It is a dielectric with a limited degreeof dioactive ionizing event is capable of releasing to 10 free electronsfrom the emitter under the action of the external electric field.

It is accordingly an object of the invention to provide.

an electron emitter requiring no. source. of power .for heating andhavinguniform emission and long life It is a further object of theinvention to provide an electron emitter havinga radioactive material.as an original energy source for initiating the .production ofelectrons.

It is another object of the. invention to provide. in such an emitter asemi-conductive material from which electronsare freed as emission fromtheradioactive material penetrates into the semi-conductive materialflIt is still a further object of the invention to sub-. ject such anemitter to the action of an electric or electromagnetic field of suchintensity and polarity to cause the electrons produced by ionization.induced by the radioactive material to move and to cause cascade ion-..ization, the field also supplying the energy for drawing all or most of'these electrons into space outside the emitter.

It is yet a further object of the invention to provide an emitter havinga readily adjustable emission under the control of an applied field.

Other objects and features of .the inventionwill be apparent when thefolowing description is consideredin connection with the annexeddrawings, in which,

Figure 1 is .atransverse cross-sectional view of one embodiment of myinvention; and

Figure 2 is a side elevation of. the device of Figure 1, portions of theouter elements being broken .away in order to show the interiorconstruction.

Referringnow to the drawings, there. is shown at.10 a supporting.element for a layer of radioactive isotope 11. As shown'in the drawings,the supportingelement loisa'metallic wire'which, in a particularinstance, was of platinumand had a diameter of eight one-thousandths(0.008) of'an inch. The supporting. element may, however, be any othermetal or conductive layer on an in-.

sulating support and, in fact, may be omitted provided that theradioactive material be conductive and rendered self-supporting alone orin combination with the semiconductor; which'may be done for examplebyforming the radioactive material in the'form of a wire, ribbon or plateor in some cases by mixingpowdered. radioactive material with 'apowderedinsulating or metallic binder and then forming it in a suitablerigidshape. by powder metallurgy. When a metallicbase is used, the metalchosen' should preferably be one which is not subject to rapidoxidization or contamination which would offer a layer of highresistance to the passage of current. Platinum is desirablefor thereason that it is highly stable and may have other materials readilycoated thereon. Other metals, such as, for example, nickel, are alsosuitable.

Plated, deposited or otherwise applied on the base wire 10 is a layer ofradioactive material 11, which material may, as has :been indicated, bean emitter of either alpha or beta particles or both. A gamma emittercan be used but is not desirable since it requires some shielding inorderto assure personnel safety. It will also cause some electricalnoise by its. action on other tube elements and envelope. In the deviceshown in the drawing, the layer 11 is preferably technetium 99 which isan emitter ofbeta particles, and this layer is approximately 0.0001 inchto 0.001 inch thick. The thickness or amount of radioactive material isnot critical. The radioactive material should be thin enough to permit avery large majority of its emitted beta particles (or alphaparticles orboth) to emerge from it and to make possible a high efiiciency ofemission per unit. area. Also, it should present low electricalresistance to current flow. Preferably an amount is applied which willproduce'suflicient initial ionization toobtain thedesired.

total emission at the operating conditions used. As has been indicated,other radioactivematerials"which emit charged subatomic particles may beutilized, the primary requirement being that the materials have arelatively long half life, values greater than one year being the mostpractical. Promethium 147, strontium 90, carbon 14 and thallium 204, inaddition to the technetium also mentioned, satisfy this requirement.

Emitters of alphaparticles havinglong-.-half lives may also be utilizedif some gamma emission can be tolerated. Examples of such emitters areradium, thorium of atomic weight of 23.0 or..232,. and uranium of.atomicWeight 238 or 234. Polonium, a pure alpha emitter, may be used inspecific cases. Oflcourse, any chemical compound of any of the foregoingelements could also be used.

Deposited or otherwise applied on the layer 11 is a layre 12 of asemi-conductive material. In a particular instance this layer 12,.was ofmagnesium oxide deposited at the-rate of 4 milligrams per squarecentimetenof the surface. of. the technetium layer ll. This thickness.was selected as large enough .to absorb substantially all thebetaelectrons emitted bythe radioactive material and toprovidesuflicient avalanche.multiplication. For otherradioactivematerials. having different maximum energy values of emittedbeta electrons, corresponding thicknesses ofsemi-conductors would beused, inaccordance with'the considerations discussed below. Anysemi-conductive material may be used, such as aluminum oxide, bariumcarbonate, silicon oxide, cadmium sulphide, selenium, germanium,cuprousoxide, zinc oxide, arsenic oxide, germanium oxide, or mixtures ofthe materials mentioned. In general, materials used for transistors mayalso be employed. 7 Each of theaforementioned materials'will exhibitdifferent efiiciencies, and even the same material will show differentefficiencies depending upon the manner in.which it is applied.Preferably, the material is applied 'so as to provide as large a surfacearea as possible, by maintaining high roughness of the outer surface. Apreferred materialris magnesium oxide, especially in a crystallineformobtained by depositing it by evaporation ina low pressure atmosphere ofoxygen of the order of 5 mm. of mercury.

Surrounding the elements 10, 11 and .12 already mentioned, isa control.grid 13 which is constructed in the manner customary in the usualelectron tube and surrounding'this may bea second similar grid 14. Thesegrids'13 and '14- are shown merely as illustrative of any number ofelectrodes which may be used to control the produced free electrons inany desiredmanner. Surrounding all of the elements mentioned is a plateor anode 15 whichis connected in the. usual manner to the positivesideof a direct voltage source, the negative side of which is connected tothe. technetium layer either through the conductive base material 10. ordirectly in cases where that base'material is omitted; Itwill beunderstood that the emitter may be at a negative potential with anaccelerating screen or grid electrode. at. ground potential, followed bycontrol and collecting electrodes, and still otherarrangementsmay beused, the. only important criterion being that electrons must besubjected to an acceleratingfield to pass outwardly through thesemiconductor andaway from the radioactive material.

The operationofthe device has been generally described; However, thedetailed operation of the embodiment of theinvention illustrated in. thedrawings will now be discussed, assuming that a'source of directpotential has been connected between the-collecting electrode or anode15 and the platinum wire 10 in the usual manner.

Betaparticles-are emitted by the radioactive layer 11, these particles.proceeding randomly in all directions. Since the base -Wire.10 isrelatively thin, the particles emitted in the. direction ofbase wire101will pass through the .wire 10 to the opposite side thereof into theradio.- activelayer 11 and semiconductor-17a on that side.. Thus all ofthe beta particles emitted by the radioactive layer particles enter thesemi-conductor layer 12 they expend their energy in ionizing themolecules thereof and releasing low energy secondary elections. Thenumber of secondary electrons liberated in the semi-conductor layer 12for each beta electron is equivalent to the energy of the ionizing betaparticle divided by the ionization voltage, and is of the order of fromto 6 thousand times the number of high energy beta particles.

The electric field resulting from the potential difference between theanode 15 and the wire 10 will cause these secondary electrons .to beaccelerated within the semi-conductor, within which they have a degreeof mobility. Secondary electrons generated at the interface of themagnesium oxide layer 12 and the technetium layer 11 will ionize furthermolecules of the magnesium oxide as they proceed from the interfacetoward the surface of the layer 12 under the influence of the appliedelectric field and will thus generate additional secondary electrons;These additional electrons are in turn accelerated and this action iscontinuously repeated resulting in a Townsend avalanche, thesemi-conductor acting like a dense gas in this respect. This avalancheproceeds outwardly until the electrons burst out from the semiconductorand become free electrons, of useful low energy, which flow to.the anodejust as in conventional electron tubes, and may be employed in the samemanner as in conventional tubes.

The electrons leaving the semi-conductor leave it positively charged atits surface, which increases the electric potential at the outer surfaceof the semi-conductor and correspondingly increases the electric fieldgradient acting within the semi-conductor to generate the avalanche.This condition arises because of the high resistivity of thesemi-conductor layer 12 which prevents the positive charge. fromdecaying immediately. Thus a relatively intense electric field isproduced within the magnesium oxide layer, the field intensity beinggreatest at the surface and least at the interface and therefore in sucha direction as to enhance the yield of secondary electrons by increasingthe acceleration of each secondary electron and thereby increasing theavalanche effect. In this way the number of emitted free electrons isincreased until an equilibrium is established.

It will be seen that if the potential at the semi-conductor outersurface were to equal or exceed the anode potential, no electrons wouldflow to the anode (except for a few of original high kinetic energy).This would soon result in dissipation of the positive surface charge,and dropipng the surface potential to an equilibrium value justsufficient to permit enough free electrons to flow to the anode tosustain the surface potential at that value. This value is estimated tobe very slightly below the anode potential, by perhaps a few hundredthsof a volt.

. The various factors which determine the resultant emission are: (l)the strength of the radioactive source, (2.) the nature and thickness ofthe semi-conductor and (3) the applied field.

The number of free electrons emitted is a function of the number ofprimary beta (or alpha) particles which is in turn dependent upon thestrength of the radioactive material. For any given radioactive materialthe ratio of free electrons to beta (or alpha) particles is a constantat any given potential ditference between the anode and base, and for agiven size and material of semi-conductor,

providing a very stable type of emitter. Additionally, the

ratio increases exponentially as the potential difference mitting use ofthe anode potential to determine the electron current up to a saturationvalue. A limiting factor may also be arcing or breakdown in thesemi-conductor, depending upon the dimensions and values used.

As indicated above, the'semi-conductor converts the higher-energyradioactive product particles into lowenergy copious free electrons. Forextremely thin semiconductors, the radioactive material particles willpass completely through without significant interaction with ithesemi-conductor, and the resultant free electrons are essentially onlythe beta particles with too high energy and too low number to be useful.As the thickness of the semi-conductor is increased, more and more betaparticles interact with the semi-conductor and in the presence of j thevimpressed electric field the avalanche or cascade action begins, andincreasing number of free electrons are produced. Since it is desirablethat the free electrons have low energy and that no beta particlesthemselves become free electrons, the semi-conductor is chosen to have athickness which will absorb substantially all beta particles emitted.from the radioactive source. Since for each radioactive source the betaparticles have a continuous energy spectrum with a well defined innerenergy limit, this can be readily done. In the case of technetium, athickness of semi-conductor providing an area density of 4 milligramsper square centimeter is suflicient. As will be recognized readily thisvalue applies for any of the semi-conductors. For other radioactivesources, their upper beta energy limits are well known, as are the areadensities necessary for absorption of those energies, and

such area densities will be used in determining the thickness ofsemi-conductor by the simple relationship that the thickness equals thearea density (mg/cmf divided by the volume density (mg./cm.

It will be understood that for such thicknesses the lower energy betaparticles will be absorbed in the initial portion of the semi-conductor.This is not important, 7

since the major portion of the total energy of the beta particlesresides in the higher energy ones.

As the thickness of the semi-conductor is increased beyond the minimumvalue required for beta absorption, a maximum free electron supply isreached, beyond which thickness the number of free electrons diminishes.This occurs for thicknesses greater than that for which the secondaryelectrons no longer have energies exceeding the work function of thematerial. It will be understood that incident beta particles haveenergies exceeding both the ionizing potential and the work function bymany, many times. The first secondaries produced are given a substantialportion of the beta particle energy and are energetic enough to producethemselves a multiplicity of other secondaries with lesser energy butstill energetic enough to produce still others. This process continueswith gradual diminishing of the secondary electron energy despite theacceleration given by the electric field, because of the highly limitedmobility of electrons in the semi-conductor. Ultimately, if thesemiconductor is too thick, the secondaries would have too little energyeither to continue the avalanching process or to become free electrons,and beyond this point increased semi-conductor thickness only serves toreduce the free electron flow. The optimum thickness is generallysomewhat greater than the minimum thickness determined by complete betaabsorption. For less than this minimum thickness, the beta particles arebeing inefiiciently utilized, and incomplete avalanching occurs, leadingto considerably reduced free electron output.

As will be seen from Fig. 1, the base 10 serves as an electrode for thereturn circuit from plate 15. To facilitate current flow, thesemi-conductor may be desirably chosen to have a contact potentialhigher than that of the radioactive material, and the radioactivematerial may have a contact material higher than that of the base.

In use, the emitter of the present invention may be used whereverconventional emitters are used and in the same .way. With gridelectrodes13 and14 omitted,

thedevicerofi'Figtl is..a.diode,.conduetive only. when. the plate ;on:anodeilSzis: positive relatiVetothe emitter base 10. In the;diode;case;.itrisinot.anecessary tohave a space between the plate and.thesemi-conductor; the plate. may be formed :fon. thesemi-conductor.surface, as by laminating,..platin'g, depositingtetc.This :.will; providea solid diode rectifier requiring-no envelope,evacuationor -gas;

filling.

For. grid controlled operationtoithesdevice:of-Fig.. 1, electrodei13.may be.used':,as':a controlzgtid and electrode.14:.as a screen Furtherelectrodes; serving. as control,v screen;:.mixer'i or; suppresser.gridswmay be;-

added.

In one preferred :form,- grid 13.. serves as a field-applying electrode;in.the; same: manner as'plate 15 was describedabove... In suchrcasegrid.13.,isyplaced close to semi-conductor Hand is madexto have a largemesh so as to capture .but-fewzelectrons. It is, made positive relativeto the emitter electrode,..andserves together with the emitter. toprovide a source of electrons in its vicinity. Grid 14 may then be.theconventional control grid, and plate 15 serves as the anode: forconventional grid-controlled operation. In this .case grid 13 ispreferably at or near ground potential, with electrode negative withrespect thereto, and the conventional cathode, con-v nections for theexternal circuit .are made to. grid.13. In effect, the cathodeisformedby the emitter plus grid 13 in suchcase. Again,.additional.electrodesserving the usual functions. inmulti-gridtubes may be added.

Although in the.drawings. no envelope has been indicated, one.wouldtnormally be supplied. It is not, however, necessary that theenvelope be evacuated or filled with any special gas under. anypredetermined pressure conditions; since theaction of electrongenerationis not affected by the atmosphere to'which' the emitter is subjected,contrary. to the conventionalzthermionic:and secondary emitters.However, Where conventional forms of electron control are used it may bedesirable to evacuate or control the pressure within the tube.

As has been indicated hereinabovethe various elements of the device maybe formedfrom a number of diiierent materials, the combinations ofmaterials utilized thus being large in number. Moreover, the exactphysical arrangementmay be considerably varied both in mannershereinabove suggested and in other Ways. The base material may beomitted, connecting the layer of radioactive material directly to thenegative terminal of the potential source when. the radioactive layer isconductive and either deposited on .aninsulating support or isselfsupporting. The shape of thesupport may be varied as desired; forexample, it may be a wire as shown, or a ribbon, or a flat. plate, or aplate which is curved, and may be covered with the-radioactive materialin part or throughout. its rarea. In each instance-it is only requiredthat the semi-conductor be juxtaposed to the radioactive source, With afield impressedfor urging electrons through and out of thesemi-conductor to become free electrons. In particular, the cylindricalconfiguration shown in Fig. 1 is not. essential; any configuration ofradioactive materialand semi-conductor may be used by which thebeta oralphaparticles or bothv are absorbed in the semi-conductor andpermitting the field to be applied thereto.

By way of example, the radioactive material 11 may be inside the base10, or on the side thereof opposite to the semi-conductor 12, so long asthe base lib does not appreciably absorb-the beta particle energy. Wherethe radioactive.;material 11 is conductive, it may itself serve as theelectrodeand the base 16 may be dispensed with.. Where the radioactivematerial id is not conductive, it may be powdered and mixed with metalpowder and then formed into a solid by pressure, a binder or otherpowder metallurgy, to form the electrode.

While the semi-conductor hasbeen described .as sufii the; electrical..noise. level within; the, tube.

ciently. thickztoabsorb all Ibetaparticls emittedby the;

radioactivemateriah.this is; not vital: This feature wields} theadvantage; that the... beta. particles; then- :do :not; be-.: come.free. electrons. .which could contribute. markedlyto ipowder mixture.

It will::be understood .that'the. theoretical explanation oh themanner..of: operatiomof the present invention given.inithe;foregoing;specification is .believed'to represent a correctstatement ,of the principles involved and the action taking placeinwthe. present invention, However, I do not wish to be limited in anyway to. or by the theory described, sinceother theories mayalso explainthe action of the invention.

Sincethe. combination of materials'aswell asthe details ofconstruotionmay be varied in the ways indicated aboveand. inmanyzotherways, .I wish to be limited not by the.foregoingdescription whichwasgiven solely for purposes of illustration, but'only by the claimsgranted to me.

What is claimed is:

1. An electron. emitter comprising a radioactive material .and means.for converting. the energy of radioaotiveparticles from said radioactivematerial into free electrons, comprising a semi-conductor inparticle-receivingassociation with saidfirst material.

2.'A.n. electron emitter comprising a radioactive material, an electrodein contact with-said material, and a semi-conductor .in contact withsaid radioactive material andseparated from said electrode.

3. An electron emitter comprising an electrically conductivexbase, alayernof a beta particle emissive material in contact therewith, and alayer of a semi-conductive material in contact with said radioactivematerial.

4. An electron emitter comprising a conductive'base, a layer of alphaparticle emissive material in contact therewith and a layer of asemi-conductive material in contact with said radioactive material.

5. An electron emitter comprising a layer of semiconductive materialdeposited on the surface of a radioactive material emitting particlesother than gamma rays.

6. An electron emitter as claimed in claim 5, characterized in than;said radioactive material is formed of at least one of the group ofmaterials comprising technetiurn, carbon 14, strontium 90, thallium 204,promethium 147, polonium, radium, uranium 234, uranium 238, thorium 230and thorium 232.

7. An electron emitter as claimed in claim 5, characterizedin that saidlayer of semi-conductive material is formed of at least one of the groupof materials including aluminum oxide, magnesium oxide, silicon oxide,germanium oxide, arsenic oxide, barium carbonate, germanium, selenium,cuprous oxide and cadmium sulphide.

8. An electron emitter comprising a semi-conductor element, aradioactive source of sub-atomic charged particles adjacent thereto forI injecting said particles into said element, and means for applying aunidirectionalfield within said semi-conductor to cause avalanching ofsecondary electrons therewithin.

9. An emitter as in claim 8 wherein said semi-conductor is formed ofmagnesium oxide and said source comprises technetium.

lOpAn electron discharge device comprising an anode and cathode, saidcathode being formed of radioactive material and means for convertingthe energy of radioactive particles fromsaid material into freeelectrons, comprising a semi-conductor in particle-receiving associationwith said first material.

11. An electron discharge device comprising 'a layer of semi-conductivematerial deposited on the surface of a radioactive-material emittingparticles otherthan gamma,

Where 1 such rays, said radioactive and semi-conductive materialsforming the tube cathode, and an anode spaced from said cathode.

12. An electron discharge device comprising an anode and a cathode, saidcathode comprising a radioactive material and means for converting theenergy of high energy particles from said material into free electrons,comprising a semi-conductive material in particle-receiving assocaitionwith said first material.

13. Electrical apparatus comprising an electrode, an anode, a source ofdirect potential connected therebetween with the positive terminalconnected to said anode, a radioactive material in contact with saidelectrode, a semi-conductive material in contact with said radioactivematerial and between said electrode and said anode whereby the electricfield between said anode and said electrode multiplies the betaparticles produced by said radioactive material by avalanching withinsaid semiconductive material.

14. Electrical apparatus comprising a support, a layer of radioactivematerial on said support, said material emitting particles other thangamma rays, a layer of semi-conductive material on said emitting layer,said layer of semi-conductive material being of a thickness to absorbsubstantially all said emitted particles, an anode spaced from saidsemi-conductive layer and a source of direct potential connected betweensaid radioactive layer and said anode.

15 Electrical apparatus comprising an electrically conductive base, alayer of radioactive material emitting alpha and beta particles, saidmaterial having a half life of at least one year, and a layer ofsemi-conductive material deposited on said radioactive material, thethickness of said semi-conductive material being such that there are 4grams of semi-conductive material per square centimeter of surface ofsaid radioactive material, said thickness being suflicient to preventpenetration of said semi-conducitve layer by said particles, saidsemi-conductive material being highly resistive to prevent rapid decayof positive charge induced in said semi-conductive material, an anode, asource of direct potential connected between said base and said anode toproduce an electric field therebetween, said electric field causing abuild-up of positive charge in said semi-conductive material therebymultiplying the electrons produced by said radioactive material byavalanching within the semi-conductive material.

16. An emitter as claimed in claim 5, wherein said semi-conductivematerial is magnesium oxide.

17. An electron emitter comprising an electrode, a radioactive materialon said electrode and adapted to produce, high-energy particles, andmeans for converting said high-energy particles into a copious supply oflowenergy free electrons, comprising a semi-conductor material inparticle-receiving relation to said radioactive material and having theproperty of producing an avalanche of low-energy electrons in responseto each of said high-energy particles impinging thereon.

18. An emitter as in claim 17 wherein said converting means furthercomprises means for applying an electric field to said semi-conductormaterial to produce said avalanche and draw said low-energy electronsfrom said semi-conductor material.

19. An electron discharge device suitable for both evacuated or gasatmosphere use, comprising an emitter formed by a source of radioactiveparticles and means absorptive of said particles for producing amultiplicity of free electrons, said means comprising a semi-conductormaterial in particle-receiving relation to said source, an electrodespaced from said source and a source of potential coupled between saidelectrode and said source, said discharge device also including at leastone further electrode beyond said first electrode.

20. An electron emitter comprising a radioactive material and means forconverting the energy of radioactive particles from said material intofree electrons, comprising a semi-conductive material inparticle-receiving association with said first material.

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